Line a baking sheet with wax or parchment paper. Place chocolate hazelnut spread in a plastic bag toward one corner of the bag. Twist so that it does not squeeze out of the top. Snip one corner off the end of the bag. Pipe small dollops of the spread onto the wax paper, about 1-1½ teaspoons each. You will need about 2 dozen. Transfer the baking sheet to the freezer and let sit until firm, about 2 hours.

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Place the butter in a medium to large skillet over medium heat. Melt the butter completely. Continue to cook, whisking frequently, until the butter foams, bubbles slightly, and begins to brown. Continue whisking until the butter is evenly browned, being careful not to burn. Remove from the heat and let cool.

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Line baking sheets with silicone baking mats or parchment paper. In a medium bowl, whisk together the flour, baking soda and salt. In the bowl of an electric mixer, combine the sugars and the brown butter. Mix on medium speed until well blended and smooth. Blend in the egg and egg yolk, scraping down the bowl as needed. Blend in the vanilla and greek yogurt. With the mixer on low speed, blend in the dry ingredients just until incorporated. Fold in the chocolate chips. Chill the dough briefly, about 30 minutes.

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Preheat the oven to 350˚ F. Use a medium dough scoop (about 2 tablespoons) to scoop a portion of cookie dough. Press an indentation into the center of the dough ball to create a bowl shape. Place a dollop of the frozen Nutella in the indentation and pinch together the edges of the cookie dough over the top to completely seal it in the center. (I then like to sort of “rough up” the surface of my cookies by making tiny pinches in the top part of the dough. It yields a more irregular, visually appealing finished product.)

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Place the shaped cookies on the prepared baking sheets, 2-3 inches apart. Bake until the cookies are golden brown and set, rotating the pans halfway through, about 14 minutes total. When the cookies are finished baking, sprinkle lightly with additional sea salt. Let sit a few minutes, then transfer to a wire rack to cool. Repeat with the remaining dough as needed (if you can resist eating it.) Be sure to replace the remaining Nutella dollops in the freezer while waiting for cookies to bake. It softens fairly quickly otherwise.